The present disclosure relates to a power supply unit and an image forming apparatus, and particularly to a technique for applying a voltage to a developer carrier.
Conventionally, there is known a technique of applying a developing roller (developer carrier) with a developing bias voltage in which a DC voltage and an AC voltage are added, so as to develop a latent image while maintaining intensity of electric field generated between a photosensitive member (image carrier) and the developing roller to be constant, and hence deterioration of image quality is suppressed. The DC voltage and the AC voltage are appropriately determined in accordance with, for example, magnetic intensity of the developer, a photosensitive material (e.g., amorphous silicon) forming the surface of the photosensitive member, and the like.
Even if an optimized developing bias voltage is applied, when a distance between the photosensitive member and the developing roller varies due to a factor such as eccentricity of the developing photosensitive member, intensity of the electric field generated between the photosensitive member and the developing roller is varied. As a result, an amount of the developer to be supplied to the photosensitive member may be excessive or insufficient.
Accordingly, conventionally, it is common to sense a capacitance between the photosensitive member and the developing roller as an indicator of the distance between the photosensitive member and the developing roller on the basis of current input to the developing roller, and to perform correction in which the developing bias voltage is decreased more as an increase of the detected current is larger while the developing bias voltage is increased more as a decrease of the detected current is larger.
However, the DC voltage and the AC voltage that are applied as the developing bias voltage are generated by transforming the AC voltage supplied from an AC power source such as a commercial power source or by converting the AC voltage into a DC voltage, and hence they are gradually stabilized at predetermined levels after a certain period of time has elapsed from start of application.
Accordingly, in the structure where the developing bias voltage is corrected on the basis of a variation of the current input to the developing roller as the conventional technique described above, when application of the DC voltage and application of the AC voltage, which are applied as the developing bias voltage, are simultaneously started, the developing bias voltage may be largely decreased for correction because an increase of the current is large during a transition period from start of the application until the developing bias voltage is stabilized at a predetermined level.
In this case, the developing bias voltage is once largely decreased in the transition period, and hence the period necessary until the developing bias voltage is stabilized may be longer than the sum of a period necessary until the DC voltage is stabilized and a period necessary until the AC voltage is stabilized when the DC voltage and the AC voltage are individually applied one by one. As a result, a period necessary until a developing operation is started may be increased.